The present invention relates to a method of forming a trailing edge cutback for one or more buckets of a turbine and particularly relates to a method of removing and refurbishing a cracked region of the turbine bucket after use to form a cutback.
Gas turbine buckets have stringent requirements for their design and manufacture. The airfoil contour of the bucket is a significant consideration in the operation of the turbine as well as during turbine repair or refurbishment, e.g., at periodic service intervals. At those service intervals, the buckets are inspected for cracking. Gas turbine buckets, particularly those constructed of a nickel-based superalloy, are sometimes prone to cracking under certain operating conditions. That is, crack initiation and propagation rates in a gas turbine bucket are typically determined by the operating conditions in which the bucket is placed.
More particularly, gas turbine buckets are conventionally cooled by flowing a cooling medium, typically air, through the buckets and which cooling air exits from the bucket through a plurality of holes spaced one from the other usually along the trailing edge of the bucket. The holes at their forward ends internally of the bucket communicate with generally radial passages carrying the cooling medium within the bucket. Generally, any involved cracking of the bucket occurs in the cooling hole nearest the root portion of the bucket along the trailing edge thereof. For example, a crack may be initiated along the trailing edge airfoil in the first cooling hole outboard from the root and directly adjacent the exit opening of the cooling hole. The crack may propagate inwardly of the bucket along the hole and across the airfoil. That is, the crack propagates at the exit hole and works its way inwardly along the hole.
In the event a crack occurs and is identified, it is essential that the bucket be repaired or refurbished before being placed back in service. Coatings are normally applied to the turbine buckets undergoing repair and coatings will often fill the crack. Thus, the crack, if left in the turbine bucket, would continue to propagate and, in most cases, could not be identified again during operation of the machine using a boroscope inasmuch as the boroscope can inspect only the external surfaces of the buckets. That is, the buckets cannot be effectively refurbished without removing or repairing the crack because, once the bucket is recoated, the crack cannot be effectively monitored for propagation. It is also significant that the airfoil configuration of the buckets be maintained to the extent possible during the refurbishing process and that airfoil-to-airfoil variation in the buckets subject to repair must be minimized to maintain long-term reliability of machine operation. It will be appreciated that generally the cracks develop along the first or second cooling hole nearest the root portion of the trailing edge of the bucket.
In accordance with a preferred embodiment of the present invention, the buckets in a turbine wheel in which cracks occur are identified and repaired, minimizing airfoil-to-airfoil variation among the repaired and non-repaired buckets wherein the repaired airfoils are refurbished only along the trailing edge from a root portion to approximately 50-75% of the span of the bucket. The repair process involves the removal of the crack at the trailing edge cooling hole nearest the root portion by machining away airfoil material around the hole and crack and creating a new airfoil geometry for the bucket at that location. The geometry gives the airfoil a trailing edge cutback from the root portion of the airfoil of the bucket to midspan. The resulting airfoil geometry minimizes the impact on unit performance caused by the removal of the material, while maintaining design requirements for thermal and mechanical stress, low-cycle fatigue and total system natural frequency.
More particularly, it will be appreciated that the trailing edge of the unrepaired turbine bucket is rounded and has a conical configuration as viewed along the length of the trailing edge. By removing trailing edge material from adjacent the midspan to the root portion and in a tapered manner, a slightly larger and new conical section is recreated along the refurbished portion of the trailing edge. To remove the material, and in one embodiment of the present invention, a planar cut is machined along the trailing edge from an intermediate span portion to the root portion, forming a planar surface along that trailing edge portion which removes the crack or cracks propagated along the cooling hole or holes nearest the root portion of the bucket. This resulting planar surface has sharp edges with the suction and pressure sides of the airfoil. Those edges are then rounded or radiussed using hand grinders and radius gauges to provide the appropriate radii at predetermined locations along the bucket span from the root portion. The resulting trailing edge is thus cut back and the cooling holes form a scalloped appearance as viewed from the sides of the bucket. It has been found that this variation in the airfoil is quite minimal upon comparison with the airfoil of an unrefurbished bucket. Consequently, it is not necessary to refurbish all buckets of a turbine wheel during repair intervals, but only those which manifest cracks. In another embodiment, a five-axis numerically controlled milling machine may be used to form the enlarged refurbished conically tapered section of the trailing edge in a single operation with the radii along pressure and suction sides being precalculated and formed at various span locations.
In a preferred embodiment according to the present invention, there is provided a method of repairing a bucket of a turbine having a crack formed along the trailing edge of the bucket adjacent a root portion of the bucket, comprising the step of machining the trailing edge of the bucket between the root portion and an intermediate span location up to about 75% of the bucket length from the root portion to form a trailing edge cutback having a conical section therealong.